CN103118979A - Aerogel and method for producing same - Google Patents

Abstract

Provided are an aerogel having excellent thermal insulation properties, an average particle diameter of about 1 to 20 μm, and a spherical shape, and a method for efficiently producing the aerogel. An airgel, the specific surface area obtained by the BET method is 400-1000m ² /g, the peak value of the pore volume and pore radius obtained by the BJH method are 3-8ml/g and 10-30nm, respectively, through the image The average particle diameter and average circularity obtained by analytical methods are 1-20 μm and 0.8 or more, respectively. In addition, a method for producing an airgel, which has the following steps in order: a step of preparing an aqueous silica sol, a step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W/O emulsion, and making the silica sol The process of converting the W/O emulsion into a gel dispersion by gelling, the process of replacing the water in the gel with a solvent with low surface tension, and the process of treating the gel with a prescribed hydrophobic agent, and a step of removing the substituted solvent.

Description

Airgel and method for its manufacture

technical field

The present invention relates to aerogels and methods for their manufacture.

Background technique

Airgel is a material with high porosity, which has excellent thermal insulation properties. The airgel referred to here refers to a solid material with a porous structure accompanied by gas as a dispersion medium, especially a solid material with a porosity of 60% or more. In addition, porosity means the value which shows the gas amount contained in an apparent volume by volume percentage. The internal heat conduction of an object depends on solid conduction (propagation of thermal vibration), convection, and radiation, and in materials with large porosity, convection generally contributes the most. On the other hand, in airgel, since the pore diameter is as small as about 10 to 100 nm, the movement of gas in the pores is largely restricted, and heat conduction by convection is significantly hindered. Therefore, aerogels have excellent thermal insulation properties.

As a method for producing an airgel, there is known a method of drying a gel compound obtained by hydrolyzing and polycondensing an alkoxysilane as a raw material in a supercritical state of a dispersion medium (Patent Document 1). In addition, the following method is also known: use alkali metal silicate as a raw material, pass it through a cation exchange resin or add an inorganic acid to prepare a sol, make it gel, and dry it under supercritical conditions in a dispersion medium (patent Document 2). Since the aerogel (silica aerogel) produced by this method has a fine silica skeleton, it exhibits excellent mechanical strength despite having a high porosity.

In the above-mentioned known production method, the dispersion medium in the gel is dried and removed under supercritical conditions, thereby suppressing drying shrinkage and replacing the dispersion medium with gas, and an aerogel having a high porosity can be produced. However, since the airgel dried under the above-mentioned supercritical conditions requires extremely high costs to realize the supercritical conditions, practical use is limited to special cases that meet such high costs. Therefore, a normal-pressure drying method for the purpose of cost reduction has been proposed (Patent Document 3).

prior art literature

patent documents

Patent Document 1: US Patent No. 4402927

Patent Document 2: Japanese Patent Application Laid-Open No. 10-236817

Patent Document 3: Japanese Patent Application Laid-Open No. 07-257918

Patent Document 4: Japanese PCT Publication No. 2002-500557

Contents of the invention

The problem to be solved by the invention

Airgel is used in a variety of applications, including use as a core material of a vacuum heat insulating material utilizing high porosity and mechanical strength, and use as an additive for heat insulating coatings utilizing excellent heat insulating properties. Of importance in such applications is the shape of the airgel particles. For example, when used as a core material of a vacuum insulation material, convection does not contribute to heat conduction, so in order to further improve heat insulation, it is important to reduce the contribution of solid conduction. By using spherical particles, the area of the contact area (contact point) between the particles can be reduced, and the gap between the particles can be increased, thereby suppressing heat conduction through the contact of the particles. Therefore, by using spherical airgel particles as the core material of the vacuum heat insulating material, the heat insulating property of the vacuum heat insulating material can be further improved. In addition, when used as an additive for paint, the filling rate can be increased by making it into a spherical shape.

As a method for producing spherical airgel, there has been proposed a method of mixing an acid and an alkali metal silicate using a mixing nozzle, spraying it, and gelling it in a droplet state (Patent Document 4). The particle size of the airgel produced by this method is determined by the size of the liquid droplets produced through the nozzle, so the particle size is about several hundred micrometers to several millimeters. On the other hand, in order to be used as a core material of the above-mentioned vacuum heat insulating material or as an additive for paint, spherical aerogels having an average particle diameter of about 1 to 20 μm are desirable. However, it is extremely difficult to obtain such micron-order fine spherical aerogels by the above-mentioned spraying method.

Therefore, an object of the present invention is to provide a method for efficiently producing an airgel having excellent thermal insulation properties, an average particle diameter of about 1 to 20 μm, and a spherical shape. In addition, there is provided an aerogel having excellent thermal insulation properties, a particle diameter of about 1 to 20 μm, and a spherical shape.

solutions to problems

In order to solve the above problems, the inventors of the present invention have repeatedly conducted intensive studies, and as a result, found that a water-based silica sol is formed, and the water-based silica sol is dispersed in another solvent that does not mix with the solvent of the aqueous silica sol to form a W/O emulsion. , and then by gelling the aqueous silica sol, replacing the solvent of the silica gel with an organic solvent, hydrophobizing the surface of the silica gel, and drying it, the above-mentioned problems can be solved, and the present invention has been completed.

A first aspect of the present invention is a method for producing an airgel, which comprises the following steps in sequence: (1) a step of preparing an aqueous silica sol, and (2) dispersing the aqueous silica sol in a hydrophobic solvent And make it form the process of W/O type emulsion, (3) make the silica sol gelation process that W/O type emulsion is converted into the dispersion liquid of gel body, (4) replace the moisture in the gel body with The process of using a solvent having a surface tension of 30 mN/m or less at 20°C, (5) the process of treating the gel with a hydrophobic agent, and (6) the process of removing the substituted solvent. The above-mentioned hydrophobic agent is compatible with A hydrophobizing agent that reacts with silanol groups present on the surface of silica and converts the silanol groups into groups represented by the following formula (2),

The silanol group is: ≡Si-OH (1)

In formula (1), the symbol "≡" represents the remaining three bonds of the Si atom,

(≡Si-O-) _(4-n) SiR _n (2)

In formula (2), n is an integer of 1 to 3; R represents a hydrocarbon group, and when n is 2 or more, a plurality of R may be the same or different from each other.

In the present invention, "hydrophobic solvent" refers to a solvent capable of forming a W/O type emulsion. "W/O type emulsion" refers to an emulsion in which water droplets are dispersed in a hydrophobic solvent.

In the first aspect of the present invention, it is preferable to gel the silica sol by adding an alkali to the W/O emulsion.

In the first aspect of the present invention, the concentration of the prepared aqueous silica sol is preferably 20 g/L or more and 160 g/L or less in terms of silicon content converted to SiO ₂ content.

A second aspect of the present invention is an airgel characterized in that the specific surface area obtained by the BET method is 400 m ² /g or more and 1000 m ² /g or less, and the pore volume obtained by the BJH method is 3 ml/g or more. And 8ml/g or less, the peak value of the pore radius obtained by the BJH method is 10 nm to 30 nm, the average particle diameter obtained by the image analysis method is 1 μm to 20 μm, and the average circular shape obtained by the image analysis method The degree is 0.8 or more.

In the present invention, "the specific surface area obtained by the BET method" means that the sample to be measured is dried at a temperature of 200°C for 3 hours or more under a vacuum of 1 kPa or less, and then, only the nitrogen adsorption side at the liquid nitrogen temperature is measured. The adsorption isotherm is a value obtained by analyzing the adsorption isotherm by the BET method. The pressure range used in the analysis at this time is a range in which the relative pressure is 0.1 to 0.25. "The pore volume obtained by the BJH method" refers to the adsorption side obtained by the BJH method (Barrett, E.P.; Joyner, L.G.; Halenda, P.P., J. Am. Chem. Soc. 73, 373 (1951)) analysis in the same manner as above. The pore volume derived from pores with a pore radius of not less than 1 nm and not more than 100 nm obtained from the adsorption isotherm. The "peak value of the pore radius obtained by the BJH method" refers to the value of the pore radius at which the largest peak is obtained in the pore distribution curve (volume distribution curve) analyzed by the BJH method in the same manner as above. The obtained adsorption isotherm on the adsorption side is obtained by plotting the differential of the cumulative pore volume obtained from the logarithm of the pore radius on the vertical axis and the pore radius on the horizontal axis.

In addition, in the present invention, the "average particle diameter obtained by image analysis method" refers to an SEM image of more than 2,000 airgel particles observed by secondary electron detection at a magnification of 1,000 times using a scanning electron microscope (SEM). Arithmetic mean value of equivalent circle diameter obtained by image analysis. Regarding each airgel particle, the "circle-equivalent diameter" refers to the diameter of a circle having an area equal to the area (projected area) occupied by the airgel particle in an image. In addition, the "average circularity obtained by the image analysis method" refers to the arithmetic mean of the circularity obtained by image analysis of SEM images of 2000 or more airgel particles observed at a magnification of 1000 times using the same SEM. value. Regarding each airgel particle, the "circularity" refers to a value obtained by the following formula (3).

$\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;S</span>}}{{\mathrm{<span\; class="notranslate">\; L</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

In formula (3), C represents circularity. S represents the area occupied by the airgel particles in the image (projected area). L represents the length (perimeter) of the outer periphery of the airgel particle in the image.

The airgel in the second aspect of the present invention is preferably hydrophobized with a hydrophobizing agent.

The effect of the invention

According to the airgel production method of the first aspect of the present invention, since the silica sol is gelled in the W/O type emulsion droplets, it is possible to efficiently produce a core material that is excellent in heat insulation and suitable for vacuum heat insulation materials Spherical aerogels having an average particle diameter of about 1 to 20 μm for various filling applications and various additive applications. In addition, the airgel of the second aspect of the present invention can be produced.

The airgel according to the second aspect of the present invention has excellent thermal insulation properties, a particle size of 1 to 20 μm, and a high degree of sphericity. The airgel has excellent fluidity and filling properties, so it is used as a core material for vacuum insulation materials. , various filling applications, additive applications, or cosmetic applications, etc. are very useful.

Description of drawings

FIG. 1 is a flowchart illustrating a method for producing an airgel of the present invention.

FIG. 2 is an SEM image of the spherical airgel of the present invention manufactured in Example 2. FIG.

FIG. 3 is an SEM image of the airgel produced in Comparative Example 1. FIG.

Detailed ways

＜1. Manufacturing method of airgel＞

The method for producing the airgel according to the first aspect of the present invention will be described. FIG. 1 is a flow chart illustrating the manufacturing method S10 of the airgel of the present invention. As shown in FIG. 1 , the manufacturing method S10 of the airgel has the following six processes in sequence.

(1) The process of preparing aqueous silica sol (silica sol preparation process S1);

(2) A step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W/O emulsion (emulsion forming step S2);

(3) A step of gelling the silica sol to convert the W/O emulsion into a gel dispersion (gelling step S3);

(4) A step of replacing water in the gel with a solvent having a surface tension of 30 mN/m or less at 20° C. (solvent replacement step S4);

(5) The step of hydrophobizing the gel with a hydrophobizing agent (hydrophobizing treatment step S5); and

(6) A step of removing the above-mentioned substituted solvent (drying step S6).

Each step will be described in order below.

(Silica sol preparation process S1)

The silica sol preparation process S1 (it may only be called "S1" hereafter) may select suitably the well-known preparation method of aqueous silica sol, and may implement. As a typical preparation method of an aqueous silica sol, the method of using an alkali metal silicate etc. as a raw material, and the method of hydrolyzing alkoxysilanes, such as tetramethoxysilane and tetraethoxysilane, are mentioned. Among these methods, a method using an alkali metal silicate is preferably employed from the viewpoint of inexpensive raw materials. Potassium silicate, sodium silicate, etc. are mentioned as this silicate alkali metal salt, and a composition formula is represented by following formula (4).

m(M ₂ O)·n(siO ₂ ) (4)

[In the formula (4), m and n each independently represent a positive integer, and M represents an alkali metal element. ]

Sodium silicate, which is readily available, is particularly preferred among the above-mentioned raw materials for preparing silica sol.

In the case of using alkali metal silicate such as sodium silicate as the raw material for preparing the aqueous silica sol of the present invention, it can be neutralized with inorganic acids such as hydrochloric acid and sulfuric acid, or the counter ion can be changed into hydrogen ion by using (H ⁺ ) cation exchange resin (hereinafter sometimes referred to as "acid type cation exchange resin"), the silica sol is prepared by replacing the alkali metal atoms in the alkali metal silicate salt with hydrogen atoms.

As the above-mentioned method of preparing silica sol by neutralizing with an acid, there may be mentioned: a method of adding an aqueous solution of an alkali metal silicate salt in an aqueous acid solution while stirring the aqueous solution; A method of collision-mixing an aqueous solution of an acid-alkali metal salt (for example, refer to Japanese Patent Application Publication No. 4-54619). The amount of the acid to be used is preferably 1.05 to 1.2 in terms of the molar ratio of hydrogen ions to the alkali metal component of the alkali metal silicate. When the amount of the acid is set within this range, the pH of the prepared silica sol is about 1-3.

In addition, the method of preparing a silica sol using an acid-type cation exchange resin can also be performed according to a known method. For example, a method in which an aqueous solution of an alkali metal silicate salt of an appropriate concentration is passed through a packed layer filled with an acid-type cation exchange resin; or, an acid-type cation-exchange resin is added to an aqueous solution of an alkali metal silicate and mixed, A method in which an alkali metal ion is chemically adsorbed to a cation exchange resin to remove it from a solution, and then the acid type cation exchange resin is separated by filtration or the like. In this case, the amount of the acid-type cation exchange resin to be used needs to be greater than or equal to the amount capable of exchanging the alkali metal contained in the solution.

As the acid-type cation exchange resin, known ion exchange resins can be used without particular limitation. For example, ion-exchange resins such as styrene-based, acrylic (ester)-based, and methacrylic (ester)-based resins can be used, and resins having sulfonic acid groups and carboxyl groups as ion-exchange groups can be used. Among them, a so-called strong acid type cation exchange resin having a sulfonic acid group can be preferably used.

It should be noted that the above-mentioned acid-type cation exchange resin can be regenerated by contacting with sulfuric acid or hydrochloric acid by a known method after being used for alkali metal exchange. The amount of acid used for regeneration is usually 2 to 10 times the exchange capacity of the ion exchange resin.

From the point of view that gelation is completed in a short period of time, shrinkage during drying is suppressed to fully form the skeleton structure of the airgel, and a large pore volume is easily obtained, the aqueous silica sol prepared by the above method The concentration of the silica component is preferably 20 g/L or more, more preferably 40 g/L or more, and particularly preferably 50 g/L or more in terms of the concentration of the silica component (SiO ₂ conversion concentration). On the other hand, from the viewpoint of relatively reducing the density of the airgel, reducing the heat conduction (solid conduction) generated by the silica skeleton itself, and easily obtaining good thermal insulation performance, it is preferably 160 g/L or less, more preferably It is 120 g/L or less, particularly preferably 100 g/L or less. By making the concentration of the aqueous silica sol more than the above-mentioned lower limit value, it is easy to make the pore volume of the airgel obtained by the BJH method 3 mL/g or more, and it is easy to make the pore volume of the airgel obtained by the BJH method The peak value of the radius is 10 nm or more. In addition, by making the concentration of the aqueous silicone gel below the above-mentioned upper limit, it is easy to make the pore volume of the airgel according to the BJH method 8 mL/g or less, and it is easy to make the volume of the airgel according to the BJH method to be 8 mL/g or less. The peak value of the pore radius is 30 nm or less.

(Emulsion forming step S2)

Emulsion forming step S2 (hereinafter sometimes simply referred to as "S2") is a step of dispersing the aqueous silica sol obtained in S1 in a hydrophobic solvent to form a W/O emulsion. That is, an emulsion is formed by using the above-mentioned aqueous silica sol as a dispersoid and a hydrophobic solvent as a dispersion medium. By forming such a W/O emulsion, the silica sol as a dispersant becomes spherical due to surface tension etc., and therefore, by gelling the silica sol dispersed in a hydrophobic solvent in this spherical shape, a spherical gel can be obtained body. In this way, through the emulsion forming step S2 of forming a W/O emulsion, an airgel having a high circularity of 0.8 or more can be produced.

As the hydrophobic solvent, any solvent may be used as long as it has hydrophobicity to the extent that it can form a W/O emulsion with the aqueous silica sol. As such a solvent, for example, organic solvents such as hydrocarbons and halogenated hydrocarbons can be used. More specifically, hexane, heptane, octane, nonane, decane, dichloromethane, chloroform, carbon tetrachloride, dichloropropane, etc. are mentioned. Among them, hexane having a moderate viscosity can be particularly preferably used. In addition, you may mix and use several types of solvents as needed. In addition, hydrophilic solvents such as lower alcohols may be used in combination (used as a mixed solvent) as long as a W/O emulsion can be formed with the aqueous silica sol.

The amount of the hydrophobic solvent to be used is not particularly limited as long as the emulsion becomes W/O type. However, generally, the hydrophobic solvent is used in an amount of about 1 to 10 parts by volume relative to 1 part by volume of the aqueous silica sol.

In the present invention, it is preferable to add a surfactant when forming the above-mentioned W/O emulsion. As the surfactant to be used, any of anionic surfactants, cationic surfactants, and nonionic surfactants can be used. Among these surfactants, nonionic surfactants are preferable from the viewpoint of easily forming a W/O emulsion. In the present invention, since the silica sol is aqueous, a surfactant having an HLB value of 3 or more and 5 or less, which represents the degree of hydrophilicity and hydrophobicity of the surfactant, can be preferably used. In addition, in this invention, "HLB value" means the HLB value obtained by Griffey's method. As described above, in the present invention, the shape of the airgel particles basically depends on the shape of the droplets of the W/O emulsion. By using a surfactant having an HLB value within the above-mentioned range, it is easy to make the W/O emulsion exist stably, therefore, it is easy to make the particle diameter of the airgel 1 μm or more and 20 μm or less, and it is easy to make the airgel particle diameter more evenly distributed. Specific examples of the surfactant that can be preferably used include sorbitan monooleate, sorbitan monostearate, sorbitan sesquioleate, and the like.

The amount of surfactant used was not changed from the usual amount when forming a W/O emulsion. Specifically, it is preferable to adopt the range of 0.05 g or more and 10 g or less with respect to 100 ml of aqueous silica sol. When the usage-amount of surfactant is large, the liquid droplet of W/O emulsion tends to become finer, and conversely, when the usage-amount of surfactant is small, the liquid droplet of W/O emulsion tends to become larger. Therefore, the average particle diameter of the airgel can be adjusted by increasing or decreasing the amount of surfactant used.

As a method of dispersing the aqueous silica sol in a hydrophobic solvent when forming a W/O emulsion, a known method for forming a W/O emulsion can be employed. From the viewpoint of easiness of industrial production, etc., it is preferable to form an emulsion by mechanical emulsification. Specifically, a method using a mixer, a homogenizer, or the like can be exemplified. A method using a homogenizer may be preferred. It is preferable to adjust the dispersion intensity and time so that the average particle diameter of the dispersed silica sol droplets falls within the average particle diameter range of the airgel of the present invention, that is, within the range of 1 μm to 20 μm. The reason is that the average particle diameter of the silica sol droplets in the W/O emulsion roughly corresponds to the average particle diameter of the aerogel. At the same time, by sufficiently reducing the particle size of the silica sol droplets in the emulsion in this way, the shape of the silica sol droplets is less likely to be disturbed, so it is easier to obtain spherical aerogels with higher circularity.

(Gelation step S3)

Gelation step S3 (hereinafter sometimes simply referred to as "S3") follows the formation of the W/O emulsion in S2 above, and gels the aqueous silica sol in a state in which droplets of the aqueous silica sol are dispersed in a hydrophobic solvent. process. This gelation can be performed using a known method. For example, gelation can be easily generated by heating to a high temperature or adjusting the pH of the silica sol to be weakly acidic or alkaline. From the viewpoint of enabling rapid gelation at low energy costs, it is preferable to cause gelation by pH adjustment.

Such pH adjustment can be easily performed by adding a base to the emulsion while maintaining the W/O emulsion formation state by stirring with the above-mentioned mixer or the like. Specific examples of the base include ammonia; tetraalkylammonium hydroxides such as tetramethylammonium hydroxide (TMAH); amines such as trimethylamine; alkali hydroxides such as sodium hydroxide; Alkali metal carbonates such as sodium bicarbonate; and alkali metal silicates, etc. It should be noted that when the pH is adjusted by an alkali metal silicate, the concentration of the aqueous silica sol described in the present invention refers to the difference between the silica source used in the preparation (S1) of the aqueous silica sol and the concentration derived from the pH adjustment. The total concentration of the silica component of the alkali metal silicate used. In addition, the intensity|strength of the said stirring should just be the intensity|strength of the grade which mixes W/O emulsion and alkali.

Among these bases, ammonia, tetraalkylammonium hydroxides, or amines are preferably used, and ammonia is particularly preferred, from the viewpoint of not mixing metal elements and requiring no washing operation. When ammonia is used, it can be blown into the W/O emulsion in the form of gas, or it can be added in the form of ammonia water. Among these, it is more preferable to add in the form of ammonia water from the viewpoint of easy pH adjustment.

In addition, the method of adjusting the pH using an alkali metal silicate has the advantage that no separate equipment for alkali is required when the same alkali metal silicate is used in the preparation of the above-mentioned aqueous silica sol.

It should be noted that the pH adjustment in the gelation step is preferably performed by measuring the amount of alkali at the target pH in advance and adding the amount of alkali to the W/O emulsion. The measurement of the alkali amount at the target pH is to take out a certain amount of sol for W/O emulsion, add the alkali for gelation while measuring the pH with a pH meter, and measure the alkali amount at the target pH. to carry out.

The time required for the above-mentioned gelation also depends on the temperature and the concentration of the aqueous silica sol. For example, when the pH is adjusted to 5 in a system with a temperature of 50° C. and a silica concentration of 80 g/L in the silica sol, from pH Gelation occurs several minutes after conditioning, and a dispersion liquid in which the gel is dispersed in a hydrophobic solvent can be obtained. It should be noted that, in the present invention, "gel" is a concept including the liquid component contained in the silica skeleton, and does not only refer to the silica skeleton produced by such gelation.

In addition, since the dispersoid changes from liquid to solid after gelation, the system is not a W/O emulsion, but a dispersion (suspension) in which a solid (gel) is dispersed in a hydrophobic solvent.

(Solvent replacement step S4)

The solvent replacement step S4 (hereinafter sometimes simply referred to as "S4") is to replace the water in the gel obtained through the above S1 to S3 with a solvent whose surface tension (γ) at 20°C is 30 mN/m or less. Step (hereinafter sometimes simply referred to as "solvent replacement", and the solvent present in the final gel may be referred to as "final solvent"). When a solvent with a high surface tension is present in the gel, drying shrinkage tends to occur during drying, and therefore an airgel cannot be obtained by non-critical drying.

(Separation and cleaning step S4-1 before solvent replacement)

Before replacing with the final solvent in S4, the gel (or the aqueous phase containing the gel) is separated from the dispersion liquid and washed as a pretreatment.

As the separation method of the gel, a known separation method for recovering the solid content from the dispersion can be used. Specifically, adding salt, applying centrifugal force, adding acid, filtering, changing the volume ratio (adding water or a hydrophobic solvent), etc. can be selected to implement one or a combination of several. Preference is given to using the addition of salt and/or changing the volume ratio. For example, by adding a certain amount of saline to the dispersion, it can be separated into a hydrophobic solvent phase and an aqueous phase containing a gel. The hydrophobic solvent phase and the water phase can be separated by a known liquid separation method such as decantation, the water phase can be recovered, and the gel contained in the water phase can be washed.

By washing the gel, impurities such as alkali metals (salts) derived from raw materials and components used for pH adjustment can be removed. Preferably, the washing of the gel is performed until the electrical conductivity of the washing liquid is 500 μS/cm or less, preferably 200 μS/cm or less.

The washing operation of the gel can be carried out by a generally known method of washing powder and granular bodies with water. For example, the following methods can be mentioned: the method of adding a certain amount of water to the gel, leaving it for a certain period of time, drawing out the washing water and repeating the operation; putting the gel into a funnel, column, etc., and passing a certain amount of water therein method etc. When washing is performed using a column, for the purpose of improving efficiency, the washing can be performed by increasing the flow rate by increasing the pressure to about 0.2 MPa to 1.0 MPa.

(Main process S4-2 of solvent replacement)

As described above, in the method for producing the airgel of the present invention, it is necessary to perform solvent replacement. This solvent substitution is to replace the water contained in the gel with a solvent having a low surface tension (final solvent) so that the gel obtained by the above method does not shrink during drying in the drying step described later.

As the final solvent, a compound having a hydroxyl group such as methanol or ethanol can also be used. However, if such a solvent having functional groups such as hydroxyl groups, thiol groups, and amino groups that are prone to nucleophilic substitution reactions, or a solvent having acidic groups such as carboxyl groups is used, there will be a possibility of hydrophobization treatment in the hydrophobization treatment step described later. In the case where a large amount of water-repellent is required because the reaction efficiency is lowered, or when a high temperature and a long time are required until the reaction is completed, there are many economic disadvantages.

Therefore, it is preferable to use a solvent that does not have the above-mentioned highly reactive functional group as the final solvent. However, solvents that do not have the above-mentioned functional groups generally have extremely low mutual solubility with water. Therefore, it is generally difficult to directly replace the water contained in the gel with the final solvent. Therefore, usually the solvent replacement includes pre-replacement and is performed in two stages. As a criterion for selecting the solvent used in the preliminary replacement, high mutual solubility in both water and the final solvent can be cited. Specific examples of the solvent used in the pre-displacement include methanol, ethanol, isopropanol, acetone, and other so-called hydrophilic organic solvents that can be mixed with water in any proportion, and methanol or ethanol is preferably used.

After the liquid component in the gel is replaced with a hydrophilic organic solvent by preliminary replacement, the hydrophilic organic solvent is replaced with a solvent having a low surface tension (final solvent). The surface tension (γ) of the final solvent must be 30 mN/m or less at 20° C., preferably 25 mN/m or less, particularly preferably 20 mN/m or less, as described above.

The following specific examples show such final solvents (the surface tension at 20°C in parentheses, the unit is [10 ^-3 N/m]): pentane (15.5), hexane (18.4), heptane (20.2), octane Alkane (21.7), 2-methylpentane (17.4), 3-methylpentane (18.1), 2-methylhexane (19.3), cyclopentane (22.6), cyclohexane (25.2), 1 -Aliphatic hydrocarbons such as pentene (16.0); aromatic hydrocarbons such as benzene (28.9), toluene (28.5), m-xylene (28.7), p-xylene (28.3); dichloromethane (27.9), chloroform ( 27.2), carbon tetrachloride (26.9), 1-chloropropane (21.8), 2-chloropropane (18.1) and other halogenated hydrocarbons; ether (17.1), propyl ether (20.5), isopropyl ether (17.7), Butyl ethyl ether (20.8), 1,2-dimethoxyethane (24.6) and other ethers; acetone (23.3), methyl ethyl ketone (24.6), methyl propyl ketone (25.1), diethyl ketone ( 25.3) and other ketones; methyl acetate (24.8), ethyl acetate (23.8), propyl acetate (24.3), isopropyl acetate (21.2), isobutyl acetate (23.7), ethyl butyrate (24.6) and other esters.

Among the solvents exemplified above, aliphatic hydrocarbons are preferable, and hexane is most preferable from the viewpoint of low surface tension and low toxicity.

In addition, among the above-mentioned solvents, if a solvent having a high miscibility with water, such as acetone, methyl ethyl ketone, or 1,2-dimethoxyethane, is used, there is an advantage that the above-mentioned pre-replacement is unnecessary.

Furthermore, from the viewpoint of being easy to dry in a drying step described later, a solvent having a boiling point of 100° C. or lower under normal pressure is preferable.

Pre-substitution can be performed using known methods. For example, a method of adding a certain amount of solvent to the gel and leaving it for a certain period of time to extract the solvent and repeat the operation; a method of putting the gel into a column and passing a certain amount of solvent through it; and the like. A method using a column is preferable from the viewpoint of saving the solvent used for the replacement. In addition, in the case of using a column for replacement, the replacement can be performed by increasing the flow rate by increasing the pressure to about 0.2 to 1.0 MPa for the purpose of improving efficiency.

The amount of the solvent used in the pre-substitution is preferably an amount capable of sufficiently replacing the moisture in the gel. The water content in the substituted gel is preferably 10% by mass or less relative to the silica component. In the above-mentioned method using a column, the above water content can be achieved by using an amount of solvent that is 3 to 10 times, preferably 5 to 10 times, the volume of the gel in the column.

In addition, of course, a further solvent replacement may be performed between the preliminary replacement and the replacement with the above-mentioned final solvent as required.

For the solvent replacement (final replacement) in which the hydrophilic organic solvent is replaced with the final solvent, it can also be carried out in the same way as the preliminary replacement, so that the solvent used in the preliminary replacement (or between the preliminary replacement and the final replacement) can be replaced. The solvent used in the further substitution) is carried out in an amount sufficient for substitution. When the method using a column is employed, the above-mentioned sufficient replacement can be achieved by using an amount of solvent that is 3 to 10 times, preferably 5 to 10 times, the volume of the gel in the column.

At the moment after the replacement of the liquid components in the gel by the final solvent is completed, due to the presence of a large number of silanol groups (Si-OH groups) on the surface of the silica skeleton in the gel, it is in a state that is easy to absorb water, In order not to contact the air for a long time until the next surface hydrophobization treatment step, it is preferable to maintain the state where the entire amount of the gel is immersed in the final solvent.

It should be noted that, from the aspect of saving the cost of the solvent, the solvent used for the above-mentioned pre-replacement, final replacement and other solvent replacements is preferably recycled and reused after purification such as distillation.

(hydrophobization treatment step S5)

The hydrophobization treatment step S5 (hereinafter sometimes simply referred to as "S5") is a step of performing a hydrophobization treatment after replacing the solvent with the final solvent. As a treatment agent for hydrophobization treatment, a silanol group capable of interacting with silanol groups present on the surface of silica is used:

≡Si-OH (1)

Hydrophobic agent that reacts and converts it into (≡Si-O-) _(4-n) SiR _n (2).

[In the formula (1), the symbol "≡" is a symbol representing the remaining three bonds (valence) of the Si atom, and does not mean that Si forms a triple bond. ]

[In formula (2), n is an integer of 1 to 3, R is a hydrocarbon group, and when n is 2 or more, a plurality of R may be the same or different from each other. ]

By performing the hydrophobizing treatment using such a hydrophobizing agent, the silanol groups on the surface of the silica are capped with hydrophobic silyl groups and deactivated, so that the dehydration condensation reaction between the surface silanol groups can be suppressed. Therefore, drying shrinkage can be suppressed even if drying is carried out under conditions below the critical point, so an aerogel having a porosity of 60% or more and a pore volume of 3 ml/g or more can be obtained. In addition, it becomes easy to set the specific surface area of the airgel at 400 m ² /g or more.

As such a hydrophobic agent, compounds represented by the following general formulas (5) to (7) are known.

R _n SiX _(4-n) (5)

[In the formula (5), n represents an integer of 1 to 3; R represents a hydrocarbon group; X represents a group (leaving group) that can be broken with the bond of the Si atom and separated from the molecule in the reaction with a compound having a hydroxyl group . When both R and X are plural, different groups may be used. ]

[In formula (6), R ¹ represents an alkylene group; R ² and R ³ independently represent a hydrocarbon group; R ⁴ and R ⁵ independently represent a hydrogen atom or a hydrocarbon group. ]

[In formula (7), R ⁶ and R ⁷ each independently represent a hydrocarbon group, and m represents an integer of 3-6. When both R ⁶ and R ⁷ are plural, they may be different groups. ]

In the above formula (5), R is a hydrocarbon group, preferably a hydrocarbon group having 1 to 10 carbon atoms, more preferably a hydrocarbon group having 1 to 4 carbon atoms, and particularly preferably a methyl group.

Examples of the leaving group represented by X include halogen atoms such as chlorine and bromine; alkoxy groups such as methoxy and ethoxy; groups represented by -NH- _SiR (wherein R and the formula R in (5) has the same meaning), etc.

When specifically exemplifying the hydrophobic agent represented by the above formula (5), trimethylchlorosilane, dimethyldichlorosilane, methyltrichlorosilane, trimethoxymonomethylsilane, triethoxy Monomethylsilane, Hexamethyldisilazane, etc. From the viewpoint of good reactivity, trimethylchlorosilane, dimethyldichlorosilane, methyltrichlorosilane and/or hexamethyldisilazane are particularly preferable.

The number bonded to the silanol group on the silica skeleton varies depending on the number (4-n) of the leaving group X. For example, when n is 2, the following combination occurs.

(≡Si-O-) ₂ SiR ₂ (8)

In addition, when n is 3, the following combination occurs.

≡Si-O-SiR ₃ (9)

In this way, the silanol groups are silylated, thereby achieving hydrophobization treatment.

In the above formula (6), R ¹ is an alkylene group, preferably an alkylene group having 2 to 8 carbon atoms, particularly preferably an alkylene group having 2 to 3 carbon atoms.

In the above formula (6), R ² and R ³ are each independently a hydrocarbon group, and preferred groups include the same groups as R in the formula (5). R ⁴ represents a hydrogen atom or a hydrocarbon group, and when R ⁴ is a hydrocarbon group, preferred groups include the same groups as R in formula (5). In the case of treating the gel with a compound (cyclic silazane) represented by this formula (6), the Si-N bond is broken due to the reaction with the silanol group, so the two The bonding occurs on the surface of the silica skeleton as follows.

(≡Si-O-) ₂ SiR ² R ³ (10)

In this way, by using the cyclic silazanes of the above formula (6), the silanol groups are also silylated to achieve hydrophobization treatment.

As specific examples of the cyclic silazanes represented by the formula (6), hexamethylcyclotrisilazane, octamethylcyclotetrasilazane, and the like are exemplified.

In the above formula (7), R ⁶ and R ⁷ are each independently a hydrocarbon group, and preferred groups include the same groups as R in the formula (5). m represents the integer of 3-6. When the gel is treated with the compound represented by the formula (7) (cyclic siloxane), the following bonding occurs on the surface of the silica skeleton in the gel.

(≡Si-O-) ₂ SiR ⁶ R ⁷ (11)

In this way, using the cyclic siloxanes of the above formula (7), the silanol groups are also silylated to achieve hydrophobization treatment.

Specific examples of cyclic siloxanes represented by the above formula (7) include hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, etc. .

The amount of the treatment agent used in the hydrophobization treatment varies depending on the type of treatment agent. For example, when dimethyldichlorosilane is used as the treatment agent, it is preferably 50 parts by weight relative to 100 parts by weight of the dry weight of silica. part or more and 150 parts by weight or less.

As for the treatment conditions of the hydrophobizing treatment, the conditions of a known method of treating the surface of silica with a hydrophobizing agent in an organic solvent may be appropriately selected according to the type of the hydrophobizing agent used. To cite an example, the following method can be enumerated: adjusting the amount of the final solvent so that the gel after the solvent replacement treatment (S4) can become a dispersion (suspension) in the final solvent by stirring. Add the hydrophobizing treatment agent, stir well, make the hydrophobizing agent spread to the whole system, and then let it react for a certain period of time, so as to proceed. For example, in the case of using dimethyldichlorosilane as the hydrophobizing treatment agent and the treatment temperature is 50° C., the reaction can be substantially completely completed by holding for about 12 hours or more.

As in the above example, even after the reaction is completed, unreacted hydrophobic agent may remain. Therefore, it is preferable to wash the gel before the next drying process to remove the unreacted hydrophobic agent. This washing removal can be performed by filtering the gel, and washing the filtered solid content once to several times with a solvent that is the same as or different from the final solvent but satisfies the requirements of the final solvent.

(Drying process S6)

Drying step S6 (hereinafter sometimes simply referred to as "S6") is a step of removing (drying) the final solvent after the hydrophobizing treatment in S5. By passing through S6, the final airgel can be obtained. The drying temperature is preferably not less than the boiling point of the final solvent used and not more than the decomposition temperature and dissociation temperature of the silyl group derived from the hydrophobic agent present on the surface of the silica, usually from room temperature to about 200°C. As for the pressure, it is preferably performed under normal pressure to reduced pressure.

In addition, when the final solvent used in S4 to S5 above has a high boiling point (low volatility), drying may be performed after replacing with a lower boiling point solvent. Of course, the solvent used for the substitution in this case must be a solvent that satisfies the above-mentioned conditions for the final solvent, that is, a solvent with low surface tension.

(Physical and usage)

The airgel obtained in this way has a spherical particle shape, and usually has an average particle diameter of 1 μm or more and 20 μm or less as determined by an image analysis method using a scanning electron microscope (SEM), and usually has a BET specific surface area of 400 m ² / g or more, or 600 m ² /g or more in some cases. In addition, the porosity shows a value of 60% or more, 70% or more in many cases, and sometimes 80% or more. In addition, the pore volume obtained by the BJH method is usually 4 ml/g or more, and often 5 ml/g or more. In addition, the peak value of the pore radius obtained by the BJH method is usually not less than 10 nm and not more than 30 nm.

In addition, in this invention, particle|grains being "spherical" means that the average circularity obtained by the image analysis method using the scanning electron microscope (SEM) is 0.8 or more. The circularity is particularly preferably 0.85 or more. In addition, the airgel particles obtained by the method for producing the airgel of the present invention generally have no corners in the particle image obtained by SEM observation at a magnification of 1000 times.

Spherical aerogels having the above physical properties can be utilized as heat insulating materials, core materials of vacuum heat insulating materials, additives for paints, cosmetics, anti-blocking agents, etc. by utilizing their properties.

In the above description of the present invention, the airgel manufacturing method S10 in which the solvent replacement step S4 is performed without aging after the gelation step S3 was exemplified, but the present invention is not limited to this embodiment. The method for producing an airgel may be a method of further including an aging step of aging the gel body following the gelation step. The specific surface area of the finally obtained airgel changes depending on the pH, temperature and time during the aging. The higher the pH, the higher the temperature, and the longer the time, the more the specific surface area of the aerogel decreases. On the other hand, since the skeleton structure of the airgel becomes stronger, there is an effect of suppressing drying shrinkage and increasing the pore volume. Therefore, the conditions may be appropriately determined according to the application of the spherical airgel to be produced, the physical property values required for the application, and the like.

The aging temperature range is preferably 30°C or higher and 80°C or lower. When the aging temperature is higher than this range, the amount of heat required to raise the temperature becomes extremely high, and when the aging temperature is lower than this range, the time required to obtain the effect of aging becomes longer. In addition, the length of the aging time is not particularly limited, and can be appropriately determined in consideration of the above-mentioned effects of aging. Among them, as a preferable range, the length of time can be exemplified as 0.5 hours or more and 24 hours or less.

In the above description of the present invention, although the method for producing airgel S10 in which the aqueous phase is separated from the dispersion of the gel in the solvent replacement step S4 is exemplified and then the gel is cleaned, the present invention does not include It is not limited to this form. The airgel production method may be a method of directly washing the gel in the dispersion without separating the aqueous phase from the dispersion of the gel. Among them, when the gel is directly washed without separating the water phase, the labor required for washing becomes extremely high because the contact between the water phases is small in the suspension state formed by dispersing the gel in a hydrophobic solvent. . Therefore, from the viewpoint of reducing the time and cost required for washing, it is preferable to wash the gel after separating the aqueous phase from the dispersion liquid of the gel as described above.

In the above description of the present invention, the method for producing the airgel S10 in which the solvent is replaced after the separation of the aqueous phase and the washing of the gel in the solvent replacement step S4 has been exemplified, but the present invention is not limited to the way. The method for producing the aerogel may be a method of directly performing solvent replacement without washing the gel body. Among them, from the viewpoint of reducing the amount of impurities such as alkali metal salts and producing high-purity airgel, it is preferable to perform solvent replacement after washing the gel body as described above.

＜2. Airgel＞

The airgel according to the second aspect of the present invention will be described.

The airgel of the present invention has a specific surface area obtained by the BET method of not less than 400 m ² /g, particularly preferably not less than 600 m ² /g. In addition, the specific surface area obtained by the BET method is 1000 m ² /g or less, preferably 800 m ² /g or less. The larger the specific surface area, the smaller the primary particle size of the silica constituting the airgel, and it is preferable to improve the thermal insulation property because the skeleton structure of the airgel can be formed in a smaller amount. Sufficient thermal insulation performance can be acquired by making a BET specific surface area more than the said lower limit. In addition, when the BET specific surface area exceeds 1000 m ² /g, it is difficult to obtain a large airgel.

The airgel of the present invention has a pore volume obtained by the BJH method of 3 ml/g or more, particularly preferably 4 ml/g or more. In addition, the pore volume obtained by the BJH method is 8 ml/g or less, preferably 6 ml/g or less. Sufficient thermal insulation performance can be obtained by making the pore volume 3 ml/g or more. In addition, it is difficult to obtain an aerogel having a large pore volume exceeding 8 ml/g.

The peak of the pore radius of the airgel of the present invention is in the range of 10 nm to 30 nm in the same analysis by the BJH method. The mean free path of gas molecules is about 100nm at 0°C and 100kPa. On the other hand, the peak of the pore diameter of the airgel of the present invention is smaller than this size, so the collision between gas molecules can be effectively suppressed. resulting in heat conduction. When the pore radius peak obtained by the BJH method is less than 10 nm, the density of the airgel increases, so the heat conduction by solid conduction increases and the thermal insulation performance decreases. In addition, when the pore radius peak obtained by the BJH method is as large as exceeding 30 nm, it becomes difficult to effectively suppress heat conduction of gas molecules, and the thermal insulation performance decreases.

The airgel of the present invention has an average particle diameter determined by an image analysis method of not less than 1 μm and not more than 20 μm. When the average particle diameter is within this range, voids of an appropriate size can be formed between the particles when filling the airgel particles. Therefore, for example, when used as a core material of a vacuum heat insulating material, excellent heat insulating performance can be exhibited.

The average circularity of the airgel of the present invention determined by the image analysis method is 0.8 or more, preferably 0.85 or more. By making the average circularity equal to or greater than the above lower limit, the area of contact points between airgel particles can be effectively reduced, so that when used as a core material of a vacuum heat insulating material, etc., good heat insulating performance can be obtained.

The airgel of the present invention having the above physical properties is preferably hydrophobized with a hydrophobizing agent. Such a hydrophobic airgel can be efficiently produced, for example, by the method for producing an airgel according to the first aspect of the present invention.

Example

Hereinafter, the present invention will be described in more detail based on Examples and Comparative Examples. However, the present invention is not limited to these Examples. Hereinafter, the BET specific surface area, the pore volume obtained by the BJH method, and the pore distribution obtained by the BJH method were measured using BELSORP-max manufactured by BEL Japan Company. The measurement of thermal conductivity was performed using HC-074-200 manufactured by EKO Instruments Co., Ltd. In addition, the bulk density was measured in accordance with the method described in JIS R1628 (constant mass method). In addition, the measurement of the average particle diameter and the average circularity is the value obtained by analyzing the image of 2000 particles using the SEM image measured by 1000 times magnification in the secondary electron detection mode. The average circularity is a value obtained by calculating the respective circularities by the above formula (3) for each of the 2000 particles observed by the above-mentioned method and arithmetically averaging the values. In addition, the average particle diameter is obtained by SEM observation of 2,000 particles using the same method, and calculating the diameter of a circle having the same area (circle-equivalent diameter) from the area occupied by the image of each particle in the SEM image. average value obtained.

<Examples 1 to 7 and Comparative Examples 1 to 2>

(Example 1)

A solution of No. 3 sodium silicate (JIS K1408) was diluted to adjust the concentrations of SiO ₂ : 80 g/L and Na ₂ O: 27 g/L. This diluted sodium silicate solution was passed through an ion exchange resin (Amberlyst IR-120B manufactured by Rohm and Haas Company) previously treated with sulfuric acid to form an H ⁺ form, to prepare 500 mL of silica sol. The pH of this silica sol was 2.8.

100 ml of this silica sol was taken out, and 0.1% ammonia water was added while measuring the pH with a pH meter, and the amount of ammonia water at which the pH became 6 was measured. Add 600 mL of hexane to the remaining 400 ml of silica sol, add 0.8 g of sorbitan monooleate, and use a homogenizer (T25BS1 manufactured by IKA) to stir for 4 minutes at 11,000 rpm. A W/O emulsion is formed. While stirring the emulsion with a mixer, 0.1% aqueous ammonia was added in an amount four times the previously measured amount to adjust the pH of the sol to 6. Stirring in this state, the liquid temperature was adjusted to 50 degreeC, and aging was performed for 24 hours. Then, 400 ml of water was added, and the water layer was separated to obtain an aqueous dispersion of a gel.

This dispersion was put into a column, and the solvent was replaced with 2 L of ethanol, and then, with 1.2 L of hexane. The gel was transferred to a beaker, hexane was additionally added to make the total volume 800 ml, and 40 g of trimethylchlorosilane was added. Then, it kept at 50 degreeC for 24 hours.

The hydrophobized gel was filtered by suction filtration, and washed with 800 ml of hexane. The gel was dried under normal pressure while flowing nitrogen gas. The drying temperature and time were 3 hours at 40°C, 2 hours at 50°C, and 12 hours at 150°C. The airgel obtained after drying was 43 g. Table 1 shows the physical properties of the airgel thus obtained.

(Example 2)

A solution of No. 3 sodium silicate (JIS K1408) was diluted with water to adjust the concentrations of SiO ₂ : 150 g/L and Na ₂ O: 51 g/L. In addition, prepare sulfuric acid whose concentration is adjusted to 103 g/L. Under the condition that the solution of sodium silicate is 1.08L/min and the sulfuric acid is 0.99L/min, each solution has a Y-shape as described in Japanese Patent Publication No. 4-54619 with a flow velocity of 10m/s or more. The silica sol was prepared by colliding and mixing in the pipe. The obtained silica sol had a pH of 2.9.

To 100 ml of this silica sol, 5% ammonia water was added while measuring the pH with a pH meter, and the amount of 5% ammonia water at which the pH became 6 was measured. Next, 600 mL of hexane was added to 400 ml of this silica sol, 1.6 g of sorbitan monooleate was added, and the mixture was stirred for 4 minutes at 11,000 rpm using a homogenizer (T25BS1 manufactured by IKA). A W/O emulsion is formed. Observe the W phase formed in the emulsion at this time with an optical microscope (magnification 400 times), and measure the particle diameter (diameter of a circle having the same area as the area occupied by the particle in the field of view of the optical microscope image) of 1000 particles, When the arithmetic mean was calculated, it was 8 μm, and the standard deviation was 1.8 μm. While stirring the emulsion with a mixer, 5% aqueous ammonia was added in an amount four times the amount measured above to adjust the pH of the sol to 6. After stirring was continued for 5 minutes in this state, 400 ml of water was added, and the water layer was separated to obtain an aqueous dispersion of a gel.

This dispersion was put into a column with a diameter of 7.5 cm, and washed with 2 L of ion-exchanged water. It should be noted that the conductivity of the washing water discharged from the column at the end was 42 μS/cm. Then, the solvent was replaced with 2 L of ethanol, and then, the solvent was replaced with 1.2 L of hexane. The gel was transferred to a beaker, hexane was additionally added to make the total volume 800 ml, and 40 g of trimethylchlorosilane was added. Then, it kept at 50 degreeC for 24 hours, and performed the hydrophobization treatment.

The hydrophobized gel was filtered by suction filtration, and washed with 800 ml of hexane. The gel was dried under normal pressure while flowing nitrogen gas. The drying temperature and time were 3 hours at 40°C, 2 hours at 50°C, and 12 hours at 150°C. The airgel obtained after drying was 43 g. Table 1 shows the physical properties of the airgel thus obtained. In addition, an SEM image (secondary electron detection mode) of the airgel at a magnification of 1000 times is shown in FIG. 2 .

(Example 3)

Dilute the solution of No. 3 sodium silicate (JIS K1408) to adjust the concentration of SiO ₂ : 75g/L, Na ₂ O : 25.5g/L, and prepare silica sol with sulfuric acid whose concentration is adjusted to 51.5g/L. Except that, an aerogel was obtained under the same conditions as in Example 2. The airgel obtained after drying was 20 g. Table 1 shows the physical properties of the obtained airgel.

(Example 4)

In the surface hydrophobization treatment step, an airgel was obtained under the same conditions as in Example 2 except that 40 g of trimethylchlorosilane was changed to 12 g of dimethyldichlorosilane. Table 1 shows the physical properties of the obtained airgel.

(Example 5)

In the surface hydrophobization treatment step, an airgel was obtained under the same conditions as in Example 2 except that the hydrophobizing agent used was changed from 40 g of trimethylchlorosilane to 14 g of methyltrichlorosilane. Table 1 shows the physical properties of the obtained airgel.

(Example 6)

An airgel was obtained under the same conditions as in Example 2 except that the condition of the homogenizer at the time of forming the W/O emulsion was 11,000 rpm for 1 minute. Table 1 shows the physical properties of the obtained airgel.

(comparative example 1)

By the same method as in Example 2, 400 ml of silica sol was prepared. The silica sol was not subjected to the W/O emulsification process (without adding hexane), and 5% ammonia water was directly added to the silica sol to adjust the pH to 6. The sol gelled in less than 1 minute. The gel was crushed moderately, and passed through a 2 mm sieve while crushing.

The pulverized gel was put into a column, and washed with 2 L of ion-exchanged water. The conductivity of the rinse water finally discharged from the column was 54 μS/cm. Then, the solvent was replaced with 2 L of ethanol, and then, the solvent was replaced with 1.2 L of hexane. After the gel was separated, hexane was additionally added to the gel so that the total volume would be 800 ml, and 40 g of trimethylchlorosilane was added. Then, it kept at 50 degreeC for 24 hours.

The hydrophobized gel was filtered by suction filtration, and washed with 800 ml of hexane. The gel was dried under normal pressure while flowing nitrogen gas. The drying temperature and time were 3 hours at 40°C, 2 hours at 50°C, and 12 hours at 150°C. The amount of the obtained dried gel was 42 g.

The dried gel was pulverized with a coffee grinder to adjust the particle diameter to a range of about 10 μm to 150 μm. The physical properties of the airgel obtained in this way are shown in Table 1 (it should be noted that the average particle diameter obtained by image analysis method is 22 μm. In addition, in the image analysis of Comparative Example 1, a magnification of 100 times was used SEM image). In addition, an SEM image (secondary electron detection mode) of the airgel at a magnification of 100 times is shown in FIG. 3 .

(comparative example 2)

An attempt was made to produce an airgel in the same manner as in Example 2, except that the drying treatment was not carried out without the hydrophobization treatment. Table 1 shows the physical properties of the obtained dried gel.

[Table 1]

＜Evaluation results＞

(Embodiments 1-6)

As shown in Table 1, in Examples 1 to 6, aerogels having a high average circularity of 0.8 or more and an average particle diameter of 1 μm to 20 μm were produced. These aerogels have a BET specific surface area in the range of 400m ² /g to 1000m ² /g, a pore volume obtained by the BJH method in the range of 3ml/g to 8ml/g, and a pore volume obtained by the BJH method. The peak value of the radius is in the range of 10 nm to 30 nm. In addition, as shown in FIG. 2 , in SEM observation at 1000 magnifications, the airgel particles are basically spherical and have no corners.

(Example 2)

As shown in Table 1, the average particle diameter of the airgel particles in Example 2 is 8 μm, which is consistent with the average particle diameter (8 μm) of the droplets of the W/O emulsion measured above. Therefore, it can be said that the shape of the droplets of the W/O emulsion determines the shape of the final airgel particles. From this result, it can be seen that by controlling the droplet diameter of the W/O emulsion, the average particle diameter of the finally obtained airgel can be freely controlled.

(Example 3)

The bulk density of the airgel of Example 3 in which the silica concentration of the silica sol was 1/2 of that of Example 2 was about 1/3 of that of the airgel of Example 2. In this way, by adjusting the silica concentration of the silica sol, the bulk density of the airgel can be adjusted.

(comparative example 1)

The airgel of Comparative Example 1 which was crushed and directly gelled without forming a W/O emulsion was difficult to control the average particle diameter in the range of 1 μm to 20 μm. In addition, since it is a fragmented type, as shown in the SEM image of Fig. 2, the shape of the airgel particles is not spherical at all. In addition, compared with Example 2, bulk density showed about 40% reduction. This is considered to be because the shape of the particles is not spherical at all as described above, and thus the packing becomes looser than that of the spherical particles.

(comparative example 2)

The gel of Comparative Example 2, which was dried without hydrophobizing treatment, had a pore volume of less than 3 mL/g, which was significantly reduced by about 60% compared with the aerogel of Example 2. In addition, the average particle size is 25% smaller than the airgel particles of Example 2. Furthermore, the thermal conductivity increased to 180% of that of the airgel of Example 2. In addition, the bulk density increased to about 150% of that of the aerogel of Example 2. It can be considered that the significant reduction in pore capacity is due to the fact that the silanol groups on the surface of the silica are not terminated with hydrophobic silyl groups, thus causing the dehydration condensation reaction between the silanol groups to cause silica drying shrinkage , As a result, most of the pores with a radius of 1 nm to 100 nm that can be measured by the BJH method were destroyed. In addition, it is considered that the pores that hinder convection are also reduced, and the density is also increased due to shrinkage. Therefore, the contribution of convection to heat conduction and the contribution of solid conduction to heat conduction cannot be suppressed, and the thermal conductivity increases.

Industrial availability

The airgel of the present invention can be suitably used as a filler such as a core material of a vacuum heat insulating material, or as an additive for a heat insulating coating. In addition, the method for producing an airgel of the present invention can be suitably used for producing such an airgel.

Claims (5)

1. A method for producing an airgel, characterized in that it has the following operations successively: (1) the process of preparing aqueous silica sol, (2) A step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W/O emulsion, (3) A step of converting the W/O emulsion into a gel dispersion by gelling the silica sol, (4) A step of replacing the moisture in the gel with a solvent having a surface tension of 30 mN/m or less at 20° C., (5) a step of treating the gel with a hydrophobic agent, and (6) a step of removing the substituted solvent, The hydrophobic agent is a hydrophobic agent capable of reacting with silanol groups present on the surface of silica and converting the silanol groups into groups represented by the following formula (2), The silanol group is: ≡Si-OH (1) In the formula (1), the symbol "≡" represents the remaining three bonds of the Si atom, (≡Si-O-) _(4-n) SiR _n (2) In the formula (2), n is an integer of 1 to 3; R represents a hydrocarbon group, and when n is 2 or more, a plurality of R may be the same or different from each other. 2. The method for producing an airgel according to claim 1, wherein the silica sol is gelled by adding an alkali to the W/O emulsion. 3. The manufacture method of the airgel according to claim 1 or 2, wherein, the concentration of the prepared described aqueous silica sol is converted to SiO with the silicon content The value of the content is more than _20g /L and 160g/L Below L. 4. An airgel, characterized in that, The specific surface area obtained by the BET method is not less than 400 m ² /g and not more than 1000 m ² /g, The pore volume obtained by the BJH method is 3 ml/g or more and 8 ml/g or less, The peak value of the pore radius obtained by the BJH method is not less than 10 nm and not more than 30 nm, The average particle size determined by the image analysis method is not less than 1 μm and not more than 20 μm, The average circularity obtained by the image analysis method was 0.8 or more. 5. The airgel according to claim 4, characterized in that the hydrophobizing treatment is carried out with a hydrophobizing agent.

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